AAT1275
Boost Converter with USB Power Switch
Due to the magnitude of the inductor ripple current,
RDS(ON)IN is the input disconnect switch, RDS(ON)N is
the high-side synchronous switch, RDS(ON)P is the
low-side synchronous switch, and RDS(ON) is the
current limit load switch.
it cannot be neglected when analyzing the RDS(ON)
power dissipation. Once the ripple current has
been determined, the RMS current during the on
and the off period can be calculated.
PCB Layout Guidelines
VO - VIN(MIN)
VO
The step-up converter performance can be
adversely affected by poor layout. Possible impact
includes high input and output voltage ripple, poor
EMI performance, and reduced operating efficiency.
Every attempt should be made to optimize the lay-
out in order to minimize parasitic PCB effects (stray
resistance, capacitance, inductance) and EMI cou-
pling from the high frequency SW node.
DMAX
=
VIN(MIN) · DMAX
L · FS
IPP
=
IO
IP =
A suggested PCB layout for the AAT1275 is shown
in Figures 1 and 2. The following PCB layout guide-
lines should be considered:
1 - D
IPP
2
IPK = IP +
1. Minimize the distance from capacitors C2 and
C3 to the IC. This is especially true for the out-
put capacitor C2, which conducts high ripple cur-
rent associated with the step-up converter out-
put capacitor.
IV = IP - IPP
2. Place the feedback resistor close to the output
terminals. Route the output pin directly to resis-
tor R2 to maintain good output regulation. R3
should be routed close to the output GND pin
and should not share a significant return path
with output capacitor C2.
3. Minimize the distance between L1 and the
switching pin SW; minimize the size of the PCB
area connected to the SW pin.
2
(IP2 + IPK · IV + IV ) · DMAX
IRMS(ON)
=
3
(IP2 + IPK · IV + IV ) · (1 - DMAX
)
2
IRMS(OFF)
=
3
PTOTAL = IRMS(ON)2 · (RDS(ON)IN
+
RDS(ON)N
)
4. Maintain a ground plane and connect to the IC
RTN pin(s), as well as the GND terminals of C1
and C2.
+
IRMS(OFF)2 · (RDS(ON)IN
+
RDS(ON)P
+
RDS(ON))
TJ(MAX)
=
P
TOTAL · θJA
+ TAMB
1275.2007.01.1.3
13
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
